I. Field of the Invention
This invention relates to a heated fuser device as is commonly used in xerographic copying machines, and more particularly to a heated fuser roll whose heating elements are formed of a material that enables the fuser to be temperature-self regulating and which includes a thermally sensitive switching device that limits excessive current surges during the initial heat-up of the fuser.
II. Description of the Prior Art
In a typical xerographic process a photoconductor comprising a photoconductive composition coated on a rigid or flexible substrate is uniformly electrostatically charged in the dark, and then exposed by being illuminated in a image pattern in accordance with graphic material on an original document. The photoconductor becomes discharged in the areas exposed to the illumination, but retains its electrostatic charge in the dark areas, which areas correspond to the graphic material on the original document. The resulting elecrostatic latent image is developed by depositing on the photoconductor a finely divided electrostatically attractable developing material (toner), e.g. a heat fusible toner. The toner will normally be attracted to those areas on the photoconductor which retain a charge, thereby forming a toner image corresponding to the electrostatic latent image. This visible image of developing material is then transferred to a support surface, such as plain paper or any other suitable substrate, to become the ultimate copy. Any residual developing material remaining on the photoconductor is cleaned and the photoconductor is reused as described above for subsequent copies. The toner image that was transferred to the plain paper is then fixed thereto. Since the developing material is heat fusible, application of sufficient heat to the paper causes the developing material to melt and be fused into the paper so as to be permanently affixed thereto.
One very basic approach to fusing in a xerographic copying machine is the use of the so-called hot roll pressure fuser apparatus. Typically, in this apparatus, the paper with the toner image thereon is passed between a pair of opposed rollers, at least one of which is heated. Generally, the heated roll is formed of a hollow cylinder having a radiant heater, such as an infrared lamp or a halogen lamp, centrally located within the the cylinder to heat the roll, in series with a bimetal thermostat. A typical example of this type of heated fuser roll is illustrated in U.S. Pat. No. 3,637,976. During operation of the fusing apparatus, the paper to which the toner images are electrostatically adhered is passed through the nip formed between the rolls with the toner image contacting the fuser roll to effect heating of the toner image within the nip. Fusing is enhanced by the second roll, or pressure roll as it is commonly called, as the result of a biasing force which forces the rolls into engagement. The thermostat intermittently interrupts the current flow as the roll temperature reaches a predetermined value. The roll then cools to some lower temperature whereupon the thermostat restores the current, and the roll heats up again.
Many of the problems that occur with the use of a hot roll-pressure fusing apparatus are located within the heated fusing roll system. In particular, these problems relate to the means employed for heating the fuser roll and its control. For example, in many of the known hot roll fuser systems it is extremely difficult to maintain a constant temperature at the nip of the rollers where the actual fusing of the toner occurs, and where temperature control is critical. Temperature control is difficult because: (1) it is difficult to sense the temperature in this region; (2) thermal lag, i.e., the responsiveness of roll temperature under varying demands of thermal output; and (3) there are both different machine modes, i.e., standby, off, continuous operation, and different size papers to contend with. The type of thermostat control as described above is conspicuously oscillatory in nature. The thermostat by necessity being situated on the circumference of the roll in order to control the temperature of that surface, is relatively remote from the heater and, thus, the temperature fluctuations are usually significant. Reductions in this aforesaid differential temperature characteristic requires extensive and expensive proportional feedback control means. In addition to these problems, radiant-type heated fuser rolls generally require very high heating temperatures for the heating element to enable the roll temperature in the nip of the rollers to be high enough to melt the toner. The use of these high temperatures can result in deterioration of the fuser roll and thus limit the life of the fuser.
Examples of miscellaneous types of heated rolls that exhibit many of the problems as outlined above are illustrated in U.S. Pat. Nos. 3,471,683, 3,720,808 and 4,100,397.
To overcome many of the types of disadvantages of hot roll fusers as described above, it has been suggested in commonly assigned and copending U.S. patent application Ser. No. 041,024, filed May 21, 1979, entitled "Temperature-Self Regulating Fuser" by Donald T. Dolan, to use a fuser member that includes a heating element formed of a material which is capable of heating the fuser member to the required fusing temperature and which is temperature-self regulating. This type of heating element is formed of a semiconducting ceramic material that has a positive temperature coefficient of resistivity and that exhibits a Curie temperature transition point at which the resistance of the material increases with increasing temperature (PTC elements). When operating a fuser containing PTC heating elements, i.e. when the fuser is first turned on, there is an initial surge of current which is undesirable. The effective resistance of the PTC material may, when cold, be of some ten ohms; on heating, it may gradually fall to eight ohms and then as the Curie temperature is approached, it will rise to more than one thousand ohms in a space of about 10 centigrade degrees. It follows that if a number of these PTC elements are required in a fuser assembly, and if these are connected in parallel, as would be desired in a heavy duty fuser embodiment (a fuser used where many copies can be made), the startup resistance could be about 2 ohms or even less. If this fuser assembly were to be used in a xerographic copier operating on a 110 volt AC supply, i.e. normal household supply voltage, a switch on current of 55 amps would occur, which is unacceptable in domestic or small office applications. From an economical and practical point of view it would be desirable to significantly reduce this initial surge of current. One solution to this type of problem is described in my commonly assigned and copending U.S. patent application Ser. No. 199,175, filed Oct. 22, 1980 now U.S. Pat. No. 4,320,284 and entitled "Heated Fuser Roll."